1. Field of the Invention
The present invention relates to a balancer shaft of an engine.
2. Description of the Prior Art
A balancer shaft of an engine is arranged substantially in parallel to a crankshaft, and normally interlocked and coupled to the crankshaft so as to be rotated at the same rotation speed as the crankshaft. A balance weight projecting in the radial direction is provided in this balancer shaft.
In such a balancer shaft structure, conventionally, the balancer shaft is formed in such a manner that one part of the balance weight passes between a pair of crank arms (crank webs) of the crankshaft. The balancer shaft is brought close to the crankshaft, and the balancer shaft is compactly arranged in the radial direction of the crankshaft.
Unexamined Japanese Patent Publication No. 04-272544 disclose an engine in which a pair of balance weights is provided on both ends of a balancer shaft in the axial direction of the balancer shaft, and a balancer shaft part between journal portions has a recess portion recessed on the axis side. Thus, the balancer shaft is compactly arranged in the radial direction of a crankshaft.
In the former balancer shaft structure, it is required to eliminate vibration due to rotation of the crankshaft by eccentric weight of only the balance weight. The balance weight is formed in a shape so as to pass between the crank arms of the crankshaft. However, the size of the balance weight in the axial direction is limited to a size that will pass between the crank arms. Thus, when the desired eccentric weight is ensured, a diameter of the balance weight has to be enlarged, and a decrease in size in the radial direction of the crankshaft is limited.
In the latter balancer shaft structure, since the balance weights are respectively provided on the outer sides in the radial direction, the size of the crankshaft in the axial direction is increased.
An object of the present invention is to provide a balancer device of an engine capable of decreasing both size in the direction orthogonal to a crankshaft and size in the axial direction of the crankshaft while sufficiently ensuring eccentric weight of balance weights by devising a shape of a balancer shaft.